The present disclosure is directed to a method and system for controlling the formation of a coating on non-line-of-sight locations on turbine engine components such as turbine airfoils.
In the pursuit of ever higher efficiencies, gas turbine engine manufacturers have relied on higher and higher turbine inlet temperatures to provide boosts to overall engine performance. In typical modern engine applications, the gas path temperatures within the turbine may exceed the melting point of the component constituent materials. Due to this possibility, dedicated cooling air is extracted from the compressor and used to cool the gas path components, including rotating blades and stator vanes, in the turbine. As a result, significant cycle penalties may be incurred.
The components of the turbine stages (e.g. vanes and blades) should be able to withstand the thermal and oxidation conditions of the high temperature combustion gas during the course of operation. To protect turbine engine components from the extreme conditions, surfaces exposed to hot combustion gases are coated with metallic bond coats that provide oxidation and corrosion resistance, and with thermal barrier coatings (TBC) that provide thermal protection. Thermal protection is provided by a low thermal conductivity ceramic top coat that decreases the heat flux into the part. Hot section part coatings are often multilayer systems that include a thermally insulating porous ceramic overlay deposited on top of various interface layers that provide thermal and environmental protection and bonding to the metal alloy substrate. The total thickness of all coatings varies between approximately 0.005 to 0.020 inches thick. TBC layers are typically made up of yttria partially-stabilized zirconia (YSPZ). The TBC has a very high melting temperature and low thermal conductivity and is resistant to oxidation and corrosion and has good phase stability. Modern TBCs can provide an extra 300° F. reduction in metal temperature; however, the TBC allows oxygen to freely diffuse through it, therefore, it does not provide any addition EBC benefit versus oxidation. In addition, the thermal expansion coefficient of the TBC should be similar to that of the alloy; otherwise the mismatch can induce cracking and spallation during thermal cyclic operation.
In order to protect the turbine components constructed from nickel-based superalloy, environmental barrier coatings (EBC), usually metallic aluminide or platinum aluminide approximately 0.002-0.004 in thick, provide oxidation and corrosion protection. The most common is NiCoCrAlY, containing 12% aluminum. Chromide coatings provide good protection for high Cr base alloys against corrosion due to molten salts, pollutants, and sands in the environment.
Aluminide bond coatings, or diffusion coatings, are a class of EBCs for nickel-based superalloys that are used in multilayered coating systems underneath ceramic overlays. TBCs are deposited on top of an alumina forming metallic bond coat that acts as an interface between the TBC and the superalloy substrate. The bond coat is used both for protection against oxidation, and to improve adherence to the metal substrate. The ceramic overlay TBC provides thermal protection while the interface bond EBC provides environmental protection to improve resistance against oxidation and corrosion. The aluminum improves the bond of the ceramic top coat to the metallic alloy substrate. The aluminide coating also acts as a reservoir of aluminum to reduce diffusion of Al to the surface. They typically are 0.002 in thick bond coat with an inter-diffusion layer approximately 0.002 in thick. The diffusion layer is non-load carrying.
Referring now to FIG. 1, there is shown a vane cluster 10 having a plurality of airfoils 12. Each airfoil 12 may interfere with the application of a coating to an adjacent airfoil(s) 12, in the regions 14. This is because the vane cluster geometry provides a mask which affects coating distribution on the hidden faces of the airfoils 12 in the regions 14, thereby reducing the amount of coating which is applied to those areas. Such non-uniformity may be further exacerbated by the absence of interference to the application of the coating on the surfaces at each end of the vane cluster. This is true even when so-called non-line of sight coating application methods, such as electron beam physical vapor deposition (EB-PVD), are used. The hidden areas may not receive adequate coating thickness due to the exposed areas on either end of the vane cluster reaching the coating thickness limit, thereby creating non-uniformity of coating profiles from vane to vane. The varying coating distribution between the two airfoils creates a situation of mismatched thermal gradients and thermal growth.
Additive layer manufacturing (ALM) devices, such as direct metal laser sintering (DMLS), selective laser melting (SLM), laser beam melting (LBM) and electron beam melting (EBM), provide for the fabrication of complex metal, alloy, polymer, ceramic and composite structures by the freeform construction of the material, layer by layer. The principle behind additive manufacturing processes involves the selective melting of atomized precursor powder beds by a directed energy source, producing the lithographic build up of material. The melting of the powder occurs in a small localized region of the energy beam, producing small volumes of melting, called melt pools, followed by rapid solidification, allowing for very precise control of the solidification process in the layer-by-layer fabrication of the material. These devices are directed by three-dimensional geometry solid models developed in computer aided design (CAD) software systems. This allows for the fabrication of very complex and multifunctional components from a wide variety of precursor materials.
Laser Chemical Vapor Deposition (LCVD) is an additive manufacturing process that a solid deposit is formed from gaseous reactants which uses a laser to heat the substrate and enhance the surface reactions in a desired location. Building off of the chemical deposition process, the LCVD pyrolytic process utilizes a non-reacting gas field to transport a target material onto a high temperature substrate. Referring now to FIG. 2, there is shown a laser CVD apparatus which includes: mass flow controller 1, precursor evaporator 2, optical lens 3, quartz window 4, substrate holder 5, and substrate 6. Referring now to FIG. 3, LCVD utilizes a laser 30 within the gas field to locally enhance surface reactions for the directed deposition of coating material along the target surface 32. This allows for controlled growth of the target material onto the substrate 32. The LCVD process has been utilized since the early 1970s to create small feature objects such as carbon nanotubes. Relative to powder bed systems such as DMLS or electron beam melting machines, LCVD utilizes a maskless process that does not need oscillatory material layering. Further, as a consequence of the gaseous material transport, the deposited material is much more pure than a powder deposition dependent process and does not suffer from deposit degradation as similar additive processes such as delamination or high porosity. Another unique deposit characteristic in LCVD is to make conformal deposits 40 on concave or convex substrates 42 results in conformal layers 44 such as shown in FIG. 4A that conform to non-planar substrates 42. In comparison, powderbed/powder deposition techniques as shown in FIG. 4B form planar layers 46 on the non-planar substrate 48.